Title:
Fuel preparation system
United States Patent 3896746


Abstract:
An improved apparatus and method using vapor generator heating gases as a medium for removing moisture from the fuel prior to it being fired in the vapor generator.



Inventors:
PIRSH EDWARD A
Application Number:
05/479067
Publication Date:
07/29/1975
Filing Date:
06/13/1974
Assignee:
The Babcock & Wilcox Company (New York, NY)
Primary Class:
Other Classes:
110/106, 122/235.11
International Classes:
C10L9/08; F23K1/04; F26B3/10; F26B17/10; (IPC1-7): F23K1/04
Field of Search:
110/104,106 122
View Patent Images:
US Patent References:
3468266FUEL FIRING1969-09-23Walker, Jr.
3246635Vapor generator with gas recirculation1966-04-19Powell et al.
3050018Pulverizer control system1962-08-21Pearson
3043525Pulverizer control1962-07-10Gilbert



Primary Examiner:
Sprague, Kenneth W.
Attorney, Agent or Firm:
Maguire, Joseph Edwards Robert M. J.
Claims:
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows

1. In combination with a vapor generator, walls defining a secondary furnace and a passageway in gas flow receiving communication therewith, a cyclone furnace discharging heating gases to the secondary furnace, means supplying heated combustion air and fuel to the cyclone furnace, said air supply means including a windbox communicating with the cyclone furnace and delivering thereto a major portion of the required combustion air, an air duct leading to the windbox, said fuel supply means including a source of high moisture content solid fuel, a crusher for comminuting the fuel, a first conduit for delivering the fuel to the crusher, said first conduit including a mix chamber, a second conduit for conveying heating gases from the passageway to said chamber for mixing with the fuel to evaporate moisture therefrom, a fan for promoting the flow of heating gases through said fuel supply means, a centrifugal separator receiving the outflow from said crusher and separating the moisture laden gases from the comminuted fuel, a third conduit for venting the moisture laden gases from the separator to said secondary furnace, and a fourth conduit receiving the fuel from the separator, said fourth conduit having an outlet discharging to the cyclone furnace and an inlet communicating with the air duct to receive heated air therefrom for further drying of and for conveying the comminuted fuel to the cyclone furnace, said last named heated air comprising the remainder of the required combustion air.

2. The combination according to claim 1 including the fan communicating with said second conduit.

3. The combination according to claim 1 including the fan communicating with said third conduit.

Description:
BACKGROUND OF THE INVENTION

This invention relates generally to the handling and burning of solid fuels and more particularly to an improved apparatus and method for drying high moisture content lignite fuel preparatory to it being fired in a combustion chamber.

In U.S. Pat. No. 3,468,266, issued on Sept. 23, 1969 to James B. Walker and assigned to the Assignee of the present invention, there is shown a system capable of successfully burning high moisture content lignite fuels in a cyclone type combustion chamber, such lignite having more than 36% moisture content and a higher heat value of less than 7,000 B.T.U. per pound as fired, and having suitable ash fusion characteristics. U.S. Pat. No. 3,468,266, while recognized as a definite improvement in the art of burning lignite coals, is directed to the exclusive use of preheated combustion air in the process of drying and conveying coal to the cyclone combustion chamber, and calls for the introduction of a first stream of preheated air into the coal stream ahead of the crusher and the use of this preheated air to dry and convey the coal to a centrifugal type mechanical separator, downstream of the crusher, where the moisture-laden air is separated from the partially dried coal. The separated moisture-laden air bypasses the combustion chamber and is vented directly to the furnace of the steam generating unit associated with the system. A second stream of preheated air is introduced at the separator particle discharge to further dry the coal and to convey the coal to the cyclone combustion chamber. This second stream of air is ultimately used as part of the combustion air required to burn the fuel. Thus in accordance with the prior art as represented by U.S. Pat. No. 3,468,266, there is required a quantity of combustion air exceeding that needed for the actual burning of the coal. This additional air having been defined as the first stream of preheated air and comprising approximately 15% of the total air being delivered to the steam generating unit.

As is well known in the art of fuel combustion, optimum efficiency is attained by limiting the quantity of excess air to no more than is actually required to completely burn the fuel, and any additional air, such as the above-mentioned first stream of air, will in fact reduce the overall efficiency of the steam generating unit by absorbing heat from the combustion gases and ultimately rejecting a portion of this heat to the atmosphere as the mixture of air and flue gases is discharged through the stack associated with the steam generating unit.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided an apparatus and method whereby vapor generator heating gases are used for drying a high moisture content fuel preparatory to it being fired in the vapor generator. The apparatus and method comprise a vapor generator including a setting formed, at least in part, with vapor generating tubes and defining a furnace and passageway means in flow receiving communication with the furnace. One or more cyclone combustion chambers are supplied with fuel and air which are combustibly mixed to generate heating gases to be passed through the vapor generator in heat exchange relation therewith. A portion of the heating gases are withdrawn from the passageway means and mixed with the fuel destined to be fired in the combustion chamber. The heating gases come into direct contact with the fuel particles and evaporate moisture therefrom. The mixture of fuel and gases is passed through one or more crushers which reduce the fuel particles to a size consistent with firing in the combustion chamber. The comminuted mixture is thereafter conveyed to a centrifugal separator for the separation of fuel and gas. The moisture-laden gases exiting from the separator are vented directly to the furnace. The dried fuel particles leaving the separator are fed into a conduit to be further dried by heated combustion air and transported to the combustion chamber to be fired therein. The main embodiment of the invention includes a fan disposed in the gas venting line discharging to the furnace. An alternate embodiment is associated with a recirculating fan used to withdraw heating gases from the passageway means for steam temperature control or gas tempering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional elevation view of a vapor generator and fuel preparation system embodying the invention.

FIG. 2 is an alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, there is shown a vapor generating and superheating unit 10 having a furnace 12 and including one or more cyclone tupe combustion chambers 14 fired with lignite fuel. Each cyclone combustion chamber 14 includes a scroll section 16 and a barrel section 18, both of which are of substantially circular cross section. The scroll section 16 is smaller in diameter than the contiguous barrel section 18 and is arranged at the front end of the cyclone chamber 14 to open concentrically into the barrel 18. The scroll includes an inlet (not shown) for tangential introduction of tertiary air. The barrel 18 has a refractory lined inner periphery backed by fluid cooled tubes and includes an inlet 20 for tangential introduction of secondary air. The rear end of the cyclone chamber 14 is partly closed by fluid cooled tubes and includes a combustion gas outlet 22 in the form of a re-entrant throat arranged therein concentric with the barrel section 18. A slag outlet 24 is formed below the gas outlet 22 for the discharge of molten slag.

One or more forced draft fans 26 supply combustion air to the vapor generating and superheating unit 10 and cause it to flow over the air heater tubes 28 and thence through a duct 30 to a windbox 32 to be apportioned among the secondary and tertiary inlets of the respective cyclone combustion chambers. Dampers (not shown) are associated with the forced draft fan, secondary and tertiary inlets to regulate the combustion air being admitted to the cyclone combustion chambers in response to fuel feed and load demand.

The lignite fuel transported by tertiary air is tangentially introduced into the scroll 16, issuing a highly turbulent flow from the scroll and causing the fuel particles, and air so introduced to move through a helical path of travel along the circumferential wall of the barrel section 18 toward the discharge end of the combustion chamber. The secondary air is admitted through the tangentially oriented opening 20 to mix with and sustain the combustion of the swirling lignite particles. The gas temperature within the barrel 18 is maintained above the fusion temperature of the ash which, when released from the burning coal, forms a molten slag covering over the barrel wall. Larger fuel particles which did not have time to complete combustion while in suspension are entrapped by the molten layer of slag and burned in situ as a result of the scrubbing action of the high velocity combustion gas. When an equilibrium slag covering of the barrel wall has been attained, the excess molten slag flows out of the opening 24 toward the bottom of furnace 12 to be discharged through opening 34 into a slag tank 36. The combustion gas exits through the re-entrant throat 22 and flows upwardly while giving up some of its heat to the tubes lining the furnace enclosure. The combustion gas is commonly referred to as flue gas after it leaves the furnace 12 to flow through the convection pass 38 containing banks of heat exchange surface defining a secondary superheater 40, a reheater 42, a primary superheater 44 and an economizer 46. The flue gas leaving the convection gas pass 38 flows through the tubes of an air heater 48 and is thereafter conveyed by a duct 50 to a gas clean-up system, i.e., precipitators, scrubbers, etc. (not shown) and discharged through a stack (not shown). A regulated quantity of flue gas is withdrawn from the convection gas pass 38, at a point either between the primary superheater 44 and the economizer 46 or between the latter and the air heater 48, to be initially conveyed through the branch ducts 52 and 54. Dampers 56 and 58 are associated with ducts 52 and 54, respectively, and provide the means where either branch duct may be used to the exclusion of the other of where both ducts may be used concurrently. It is to be understood that the source of flue gas and the location for the withdrawal thereof may be other than that shown herein.

The embodiment depicted in FIG. 1 includes a fan 60 integrated into the lignite drying system to provide the motive force required to convey the flue gas therethrough, whereas, the lignite drying system of the embodiment depicted in FIG. 2 is serviced by a fan 62 associated with the vapor generating unit gas recirculating system; this system being used to temper the furnace combustion gas or to control the final steam temperature and including a duct 64 equipped with a damper 66 which regulates the quantity of flue gas being delivered to the plenum chamber 68 for introduction into the furnace 12 through openings (not shown).

Referring again to FIGS. 1 and 2 and particularly to the lignite drying system, we have a damper 70 preferably situated at or near the inlet of a conduit 72 for regulating the quantity of flue gas being admitted thereto. The flue gas passing through the conduit 72 is delivered to a mix chamber 74 for direct contact with the lignite thereby absorbing a portion of the moisture content therein.

The high moisture lignite is normally stored in a bunker 76. When the shut-off valve 78 is open, the lignite dicharges via conduit 80 onto feeder 82. The fuel feed-rate to the firing system is controlled by regulating the feeder speed. The lignite leaving the feeder 82 is conveyed through conduit 84 into the mix chamber 74 where it is mixed with flue gas discharging from the conduit 72. The flue gas performs a dual function of drying the lignite and tranporting it through a portion of the drying system. The fuel and gas mixture travels through the conduit 86 into a crusher 87 where it is comminuted to a size consistent with firing in a cyclone type combustion chamber. The crushed fuel is swept out by the accompanying flue gas and carried through a conduit 88 to a centrifugal type separator 89 wherein the moisture-laden flue gas is separated from the lignite particles. The flue gas is vented through a conduit 90 into the furnace 12. The rate and pressure of the vented discharge is regulated by a control damper 91. A pair of tight shut-off dampers 92 is provided in conduit 90 to prevent the backflow of furnace gases when the system is shut down, at the same time, seal air 93 is introduced between the dampers 92 to further seal the conduit 90.

The embodiment depicted in FIG. 1 includes the fan 60 flow connected into the conduit 90 at a point intermediate the control and shut-off dampers 91 and 92.

In the embodiments shown in FIGS. 1 and 2, the partially dried lignite outflow from the separator 89 is discharged into a rotary seal feeder 94 from whence it is fed into a mixing T 95 to be swept up by heated combustion air for discharge into the scroll section 16 of the cyclone combustion chamber 14. The duct 30 supplies heated air for the conduit 96, the latter including a regulating damper 97 to control the air flow therethrough and tight shut-off dampers 98 situated at the conduit inlet and outlet ends to isolate the system when it is shut down and during which time seal air 99 is introduced between the dampers 98 at the conduit outlet to further seal the conduit 96 from the backflow of combustion gases. The heated air normally conveyed through conduit 96 acts as a transport medium while also having the function of further drying the lignite particles before introduction into the cyclone combustion chamber.

While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.